This invention relates to FM stereophonic broadcasting systems and, more particularly, to a receiver for an FM stereophonic broadcasting system which increases the broadcast coverage area over that of current biphonic service which utilizes circuits for reducing the effects of multipath on the received signal.
U.S. Pat. No. 4,485,483 describes a stereophonic broadcasting system incorporating companding of the difference signal which is compatible with existing receivers and which through signal-to-noise improvement significantly extends the area of coverage of an FM stereo station. In this system, and as illustrated in FIG. 1, stereophonically-related audio frequency source signals L and R are matrixed to obtain stereophonic sum and difference signals M and S, respectively. At the transmitter, the difference signal is used to amplitude-modulate a first subcarrier signal and at the same time is applied to a compressor which compresses its dynamic range to produce a compressed difference S'. The compressed signal S' is used to amplitude-modulate a second subcarrier signal of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, double-sideband modulation is employed, with the frequency of the sub-carrier signal being sufficiently high to assure a frequency gap between the lower sidebands of the modulated sub-carrier signals and the M signal. A conventional low-level phase reference pilot signal, lying within the aforementioned frequency gap, is provided for detection purposes at the receiver. The M signal, the two modulated sub-carrier signals, and the pilot signal are frequency modulated onto a high frequency carrier for broadcast. The receiver includes a demodulator for deriving the M signal, the normal difference signal S and the compressed difference signal S', and an expander for expanding the derived compressed difference signal. The expanded noise-reduced version of the difference signal is combined with the received sum signal to obtain the original audio frequency source signals L and R. In addition to improving the quality of the received signal, the system increases the broadcast coverage area over that of current biphonic service.
Commonly assigned U.S. Pat. No. 4,602,381, the disclosure of which is hereby incorporated herein by reference, describes the use of the difference signal S as a reference for controlling the expansion of the received compressed difference signal S' so as to cause the amplitude of the expanded difference signal to equal the level of the uncompressed difference signal, making the expander pressed difference signal, making the expander adaptive to any compression characteristic that might be employed at the transmitter. This adaptive expander performs very well under stationary conditions, that is to say, when there is no relative movement of the receiver with respect to the transmitter.
However, in mobile applications, that is, with the receiver mounted in a moving vehicle and thus more prone to disturbances caused by the multipath propagation phenomenon, a condition in which a receiving antenna is sensitive to both a direct transmitted signal as well as to multiple, delayed reflections of that signal caused by terrain factors or man-made structures, the normal difference signal (L-R) may be adversely affected so as to no longer be suitable for controlling the expansion of the received compressed difference signal S'. Depending on delay intervals, multipath propagation can decrease the level of the received RF signal so as to cause noisy reception or complete signal dropouts. In conventional stereo receivers, the effect is characterized by momentary bursts of noise as the vehicle moves through the multipath space. In addition to this RF signal fading, the summation of the multipath signals at the receiver may also distort the phase relationship between the pilot signal and the stereo difference signal, and with the inclusion of the added compressed difference signal S', such momentary phase errors can result in momentary bursts of crosstalk as well as noise. Since the level of the compressed difference signal is generally higher than that of the normal difference signal, if the phase error is such as to cause crosstalk summation of the two stereo difference signals, loud bursts of sound may be heard. For these reasons, in the presence of multipath the resultant expanded difference signal will be corrupted by reason of its being controlled by a corrupt (L-R) reference signal.
A primary object of the present invention is to provide an improved receiver for an FM stereophonic broadcasting system incorporating companding of the difference signal which reduces the adverse effect of multipath disturbances on the reproduced stereo signal.